Document entryPoint and idls options in as-thrift docs

[andrewdeandrade]

The entryPoint option needs to be documented in the as-thrift documentation. Specifically, we need to call out that relative paths only work with the entryPoint documents.

We should also document the idls option that allows the user to read all the sources themselves and provide them in an object where the keys represent the relative path.

Lastly, it might be worth having a better error when trying to load a file with the source option when the source itself contains relative includes.

@kriskowal

[rajeshsegu]

We have built a library where we feed in the source of the idl file, but all the relative includes fail. I think entryPoint attribute could be useful here. I will try to add documentation in case you guys are ok.

[andrewdeandrade]

@rajeshsegu yes, a pull request with documentation would be much appreciated.